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I have a row house with a "flat roof" meaning it actually has a small grade so that water runs off the back. However, towards the middle of the roof there is a depression that allows an inch deep puddle to form after a rain. I believe the depression is caused by a the settling of the roof from the weight of the roof deck. We have recently had a leak due to the puddle and so I did a fairly thorough job patching and putting a fresh layer of roofing cement down in the entire area that collects a puddle that is about 2 feet by 6 feet. It is a rolled rubber roof. I understand that all this may mean I should probably start planning on replacing the roof in the future but I'm looking for a medium term improvement for the puddle.

I was talking to the hardware store employee and he suggested putting play sand down fill the depression and then put a layer of the rolled rubber down to flatten it out. That seems like a reasonable way to fill the irregular form of the space but does that seem like an ok medium term solution?

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  • Your best course of action will be to determine why there is a depression in the first place and correct that issue. That may involve removing at least some of the roofing material and then replacing or patching it. I think that adding something like sand will just increase the problem.
    – jwh20
    Aug 23, 2021 at 12:53
  • The roof decking(plywood,OSB) should not have a depression. There is a problem(major or minor) that needs to be fixed first, instead of trying to hide it. If it is caused(unlikely) by the weight of the roof, you can have a major problem.
    – crip659
    Aug 23, 2021 at 13:22
  • When wind blows across your roof, it leaks under the edges of your rubber roofing and will move grains of sand. You probably have prevailing winds in your area so the sand will eventually end up on the leeward side of your roof.
    – Lee Sam
    Aug 23, 2021 at 14:12
  • if you're going to fill a depression with sand. you'll need to add some sort of binder to hold the sand together so thart it does not shift arond under the rubber membrane, it might be easier to just use instant asphalt to fill the depression, and then rubber over that. but you shoulf probably go looking for the cause and fix that instead.
    – Jasen
    Aug 23, 2021 at 14:26
  • The weight on that depression by filling it with sand will be there 24/7/365 and will likely make that depression worse a lot sooner than just fixing the leak and living with the puddle - the puddle will at least dry up periodically and reduce the weight on the damaged area. If you could find something very light-weight to fill the depression, then maybe consider that idea - but really the underlying problem needs to be fixed.
    – brhans
    Aug 23, 2021 at 15:48

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If the roof is sagging due to moisture from a slow, long-term leak softening the OSB/ply subroof, or due to damaged ceiling joists, then the added weight of sand is only going to make it worse.

I think that the general consensus of the comments so far is that if your patch is actually sealing out the water, that's probably the best short- to mid-term fix you can do, and that the long-term solution of finding and fixing the cause of the depression, along with a professional level patch or reroof is really your next best step.

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If it were my house and I knew I wouldn't be able to strip and replace the roof for a year or so, I would mark the outline of the 2 x 6 foot puddle with chalk or spray paint, wait for a dry day, then lay down a 4 x 8 sheet of 1/2" plywood to cover the depression. Screw it down well, apply a liberal coating of roofing cement and start planning a proper repair. Water won't collect there any more.

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